PARIS (Reuters) - Defeat is never an option for Serena Williams, but the world number one came close to an embarrassing second-round exit at the French Open after an 'unprofessional' display on Thursday.

The American eventually beat unheralded German Anna-Lena Friedsam 5-7 6-3 6-3 to set up a meeting with 27th seed Victoria Azarenka of Belarus, but this was one of her off days,

She hit a flurry of unforced errors in the first set and faced two break points in the first game of the second before regaining her composure and reeling in the mistakes.

Williams, whose 19 grand slam titles include the French Open crowns in 2002 and 2013, has experienced spectacular failures here before, having been knocked out in the first round in 2012 and in the second round last year.

Another humiliation loomed on court Suzanne Lenglen as the 33-year-old struggled with her foot movement and ball striking and her serve misfired badly.

"I definitely don't think it could go worse than that," she said. "I'm just happy I was able to get through that, because I have played some horrible matches and lost them.

"I honestly thought for a second, I wasn't sure I could win today because the level I played was not professional. So I was quite concerned about that. Being able to get through that really is pretty good, I guess."

When she broke Friedsam's serve at the start of the third set the chance of an upset began to fade as the German's belief finally withered. Williams broke serve again to claim victory.

"I never set the bar low for myself. That means I accept defeat -- and I never accept defeat," she said.

"I think you have to be mentally ready and prepared for anything. And I'm not ever going to put myself in a position where I say I'm not good enough, because I know I am."

The bar will be higher against former world number one Azarenka, who she beat in three sets in the round of 16 at the Madrid Open earlier this month.

But Williams, who said she could not practice her serve properly because of a wrist injury, does not feel any pressure as she aims at a 20th grand slam title.

"I just feel like now I don't have anything to lose either. Like if I'd have lost that match I still would have Wimbledon in a few weeks," she explained.

"So it's a different mindset for me, as well. I still have time. If I don't get it, I don't think I'm going to be very depressed about not reaching 20 when you have 19."